I cannot send mail today for hours

I cannot send mail today for hours

Hi,

I am a Zoho mail user. I host my domain name mailbox at Zoho.

I found that I cannot send mail today for hours. If I use webmail to send mail, the sent mail can be shown in the sent folder, but the receiver cannot get the mail (even not in his Spam folder). If I use an SMTP client to send mail, the sent mail will not be shown in the sent folder on the webmail, and the receiver cannot get the mail neither.

Please help me to solve the problem. Thanks!